Evidence against a potential endoplasmic reticulum transmembrane domain of 27K zein expressed in Xenopus oocytes.

During the cellular protein targeting process, zeins (maize storage proteins) are retained in the endoplasmic reticulum (ER) where they accumulate into protein bodies. There are circumstantial and preliminary data indicating that the 27K zein, a class of zein proteins, may span the ER membrane. This potential transmembrane domain is considered very significant with regard to the mechanism of ER retention for zeins. The potential transmembrane domain may permit the 27K zein to remain in the ER and to serve as an anchor for other classes of zein, thus acting as a nucleating factor for protein body formation. This study investigated the potential transmembrane feature in a heterologous system (Xenopus laevis oocyte). Following injection of synthetic 27K zein mRNA, isolated protein vesicles were subjected to proteinase K digestion, surface biotinylation and alkaline extraction. Throughout three categories of assay the possible role of the 27K zein as a transmembrane protein was consistently refuted in this study. The 27K zein polypeptide was affected by neither proteinase K digestion nor biotinylation and was released from alkali-stripped membranes. This study, therefore, concludes that the 27K zein is not a protein body nucleating factor by virtue of an ER transmembrane feature.